As this sort of exhaust gas measurement system, as disclosed in JP-A2003-50194, there is one adapted to dilute the exhaust gas of an internal combustion engine at a predetermined dilution ratio to conduct the diluted exhaust gas to a filter, and measure the mass of particulate matter (hereinafter also referred to as PM) in the diluted exhaust gas collected by the filter.
Meanwhile, in a recent PM measuring test, weighting a measurement result depending on a vehicle running mode has been legislated, and for example, the CFR 1066 stipulates that a measurement result in the first running mode (cold start) is given a weight of 0.43, a measurement result in the second running mode (stabilized) is given a weight of 1, and a measurement result in the third running mode (hot start) is given a weight of 0.57.
As such a weighting method, there is a method that measures the mass of PM using one filter for each of the running modes, and multiplies the resulting measurement value by a corresponding one of the above-described weighting values. However, in recent years, PM concentration in exhaust gas has been reduced, and the amount of PM collected by a filter in each running mode is extremely small, making it difficult to accurately measure the mass of the PM.
On the other hand, there is an approved method that weights the flow rate of diluted exhaust gas in each of the running modes, and uses a filter common to the respective running modes to collect PM.
However, if a weighting value is small, this method gives rise to the problem that the diluted exhaust gas flow rate and a diluent gas flow rate fall below the measurement range of a venturi flowmeter suitable to measure each of those flow rates to prevent the respective gas flow rates from being accurately controlled, and therefore the dilution ratio accuracy of the diluted exhaust gas cannot be secured.